An EDM process is carried out by utilizing a tool electrode, e.g. in the form of a solid body or an axially traveling wire, spacedly juxtaposed with a conductive workpiece electrode across a machining gap filled with a liquid dielectric, e.g. a hydrocarbon or distilled water, and applying a series of electrical discharge pulses through the gap between the electrodes to electroerosively remove material from the workpiece electrode. As material removal proceeds, a machining feed must be effected between the tool and workpiece electrodes, by advancing one of them relative to the other, to continue material removal or machining in the workpiece while maintaining the machining discharge gap substantially constant. It is critical that the machining gap have a size as small as 10 .mu.m (micrometers) or so. To maintain such a small gap, the movable electrode should be controlledly advanced to follow material removal but not to overtravel so as to cause a short-circuiting condition in the gap.
It has been known that a preferred EDM machining feed system hitherto proposed to this end makes use of a stepping motor drivingly coupled with the movable electrode and energized with a succession of drive pulses to advance the movable electrode incrementally or in successive steps. It has experimentally been found to be desirable that the movable electrode should advance a fixed incremental distance d as small as 1 .mu.m (micrometer) or less to achieve best results. A desired overall advance displacement of distance D can thus be accomplished by n repetitions of the incremental displacement d, n=D/d, and hence by energizing the motor with n uniform drive pulses of a duration .tau. which defines the incremental displacement d.
With the prior system, however it has now been found that in an EDM machining feed the movable electrode for each required incremental advance, especially where the increment d set is as small as 1 .mu.m, does not always respond quickly and accurately to a commanding drive pulse. As a consequence, each commanding drive pulse may actually result in a displacement shorter than the desired increment, or the overall advance may become continuous, rather than stepped as desired, and may not bring the movable electrode to a precise position as commanded by the sequence of incremental drive pulses applied to the motor. Thus, there may result a serious inaccuracy in the position reached in each machining feed block. Furthermore, the failure in stepping feed may cause overtravel of the movable electrode, tending to cause a short-circuiting condition in the gap and a failure in machining stability.